A conducting medium, such as a metal, a semiconductor, or a plasma, reflects electromagnetic waves impinging on its surface, as is attested for by the shiny appearance of metals. This well-known phenomenon is due to currents induced in the conducting medium by the incoming wave, and the emission of an outgoing wave by these currents acting as an antenna.
Under special circumstances, a different kind of reflection can occur, namely phase-conjugate reflection (R. Fisher, editor: Optical Phase Conjugation. Academic Press 1983. B. Ya. Zeldowich, N. F. Filipetsky and V. V. Shkunov: Principles Of Phase Conjugation. Springer 1985). Whereas regular reflection preserves the phase of the incoming wave, up to a costant .pi., phase conjugated reflection inverts it. As a result, regular reflection obeys Snell's Law: the perpendicular to the reflecting surface bisects the angle between the incoming and the outgoing rays. In phase-conjugate reflection, on the other hand, the outgoing ray retraces the incoming ray in reverse, i.e. returns toward the radiation source. When several rays emanating from a point impinge on a conventional reflector, they are so reflected as to further diverge; a phase conjugate reflector converges the rays back to their origin.
These phenomena are by now well established in the field of optics, where special materials have been developed, which serve as phase conjugate reflectors for visible and infrared light. The present invention provides means for phase conjugate reflection of far infrared, microwaves and millimeter waves, i.e. a range where this phenomenon has not been established before. In particular, it is shown that plasma, either in the form of high density of charge carriers in crystals, or in high temperature ionized gases, can be utilized to construct phase conjugate reflectors for electromagnetic fields in these regimes. The reflection of these waves can also be amplified in intensity, as is described below. As a result, it is possible to construct a radiation source similar to the laser, namely of high intensity, coherence and directionality, in the above mentioned regimes.
Several possible fields of application are described for this device, which will hereafter be called plasma phase conjugate reflector, or PPCR.